A detailed review was made of data pertinent to the occurrence of chronic Chagas disease in the U.S.A. In Latin America, 16 to 18 million individuals are thought to have Chagas' disease and 90 million are considered to be at risk of infection. In the U.S., the occurrence of Chagas' disease is virtually limited to individuals who have resided in Latin America where they acquired the infection, and then migrated to this country. Data on the prevalence of positive serologic reactions for Tripanosoma cruzi serve for calculating that a total of up to 74,000 Latin Americans residing in the U.S. have the chronic form of chagasic cardiomyopathy. The vast majority of these individuals are either undiagnosed, or misdiagnosed as having idiopathic dilated cardiomyopathy or coronary artery disease. Vector transmission of T. cruzi infection is very unlikely to occur in the U.S. because of variations in biological behavior of local species of insect vectors and because of changes in human living conditions. Transfusion of blood from infected but asymptomatic individuals is considered the most important mechanism of transmission of this disorder in the U.S.